As is commonly known, melamine can be prepared by evaporating liquid or solid urea and introducing the resulting gases, along with a carrier gas, into a fluidized catalyst bed having a temperature of between about 300.degree. and 450.degree. C. It is also known that melamine can be prepared by introducing solid or liquid urea directly into a catalyst bed fluidized with an inert gas and/or ammonia, and having a temperature of between about 300.degree. and 450.degree. C., so that the catalyst particles become loaded with urea. The urea is then thermally decomposed and the resulting decomposition products converted into melamine. The resulting gaseous mixture containing the melamine is then discharged and the melamine is isolated and recovered in a known way.
It has also been proposed to carry out this reaction in a reactor having two separate beds of fluidized granular material. According to U.S. Pat. Nos. 3,158,611, 3,095,416 and 3,152,128, urea is evaporated in a first zone and decomposed in a bed of material that does not promote the conversion of the resulting urea decomposition products into melamine under the reaction conditions. The resulting vapor is then fed into a second zone containing fluidized catalyst, in which zone the conversion into melamine is effected. A variation of such a two-zone process is disclosed in U.S. Pat. No. 3,332,947 wherein, in the first zone, the granular catalyst is loaded with urea and decomposition products thereof at a comparatively low temperature, whereafter this laden catalyst is fed into a second zone maintained at a higher temperature in which the conversion into melamine is effected. However, none of the above known two-zone processes obtain a yield even approaching 95%.
According to Netherlands Patent Application No. 6,503,913, the reaction is effected in a first reaction zone at a temperature of between 370.degree. and 450.degree. C., and in a second reaction zone at a temperature of between 330.degree. and 370.degree. C., and a comparatively small part of the catalyst is periodically or continuously exchanged between the two zones. It is claimed that in this way the activity and selectivity of the catalyst are maintained for a longer period. The yields of such a process are at most 95%.